Modeling of the influence of H2O on metal oxide sensor responses to CO

In this work a model able to predict the dynamic response of conductimetric SnO2 sensors in presence of humid mixtures of a reducing gas and oxygen is presented. Sensors consisting of large-grained porous films are taken into account. For these sensors the conduction is given by the surface charge carriers, whose density is exponentially related to the height of the potential barrier that establishes at the surface of the grains. The barrier is due to the presence of some localized charge, trapped both on surface intrinsic defects and on chemisorbed species. In this perspective the complete dynamic model of sensor response is obtained by combining a set of differential equations describing the kinetics of surface species adsorption and ionization, the relationship between the density of the charge trapped at the surface and the potential barrier magnitude, and, finally, the relationship between film conductance and the potential barrier height. The presented work is a development of some previous studies that led to the development of a model of the sensor behavior in dry mixtures of oxygen and CO. In the present study the influence of water on sensor response, in the same operating conditions previously considered, is treated, and a simple model of its effect is proposed. Water is accounted for as a further adsorbant interacting with the other two chemical species. A dissociative adsorption is considered, where adsorbed OH− groups, by loosing their electrons, behave as donors and inject a non-negligible (significant) amount of free electrons in the conduction band. The experimental results obtained with a set of 8 Taguchi sensors prove that the proposed model can predict the most important characteristics of the sensor behavior.